Conventionally, there have been known premixed compression ignition engines as engines that compress fuel under high pressure in a lean state and lead to self-ignition and combustion, thereby achieving the small amount of smoke and the small amount of NOx. The premixed compression ignition engine is configured to compress air-fuel mixture under high pressure in a combustion chamber and carry out self-ignition and combustion.
The premixed compression ignition engine is required to uniformly and leanly mix the fuel with intake air. Also, even when the fuel and the intake air are uniformly, leanly mixed, there is a high possibility that knocking or an accidental fire occurs due to change in terms of an elapsed time from the start time of start-up operation, engine load, an air-fuel ratio, and the like. Regarding the aforementioned premixed compression ignition engine, there has been known an engine in which exhaust gas (EGR gas) is mixed with the intake air at a predetermined timing based on the operational states of the engine, thereby suppressing the occurrence of the knocking or the accidental fire and appropriately controlling the state of combustion. For example, Patent Literature 1 discloses the above-mentioned engine.
However, in the premixed compression ignition engine disclosed in Patent Literature 1, the EGR gas mixed with the fuel substantially fluctuates in temperature thereof or constitution of components of the EGR gas, depending on the operational states of the engine. Accordingly, there is a disadvantage in that it is difficult to suppress the occurrence of the knocking or the accidental fire only based on the timing (quantity or density) of mixing the EGR gas.